Da is on the right track. Italian is very overrated. Most people who say Italian food is their favorite are chowing down at Olive Garden and Pizza Hut. Very good Italian food is obviously great. It's still way overrated. So is French food. It's fine. Everyone loves great cheeses and breads. Everyone also loves 30 pounds of butter. There are obviously many incredible French restaurants, but too often French is just a stand in for something that seems like it should be fancy.
The most underrated is clearly the US of A. I could have a long post about this, but just know that if you disagree with me, you are wrong.
I also probably shouldn't have made it 3 because the top 2 are obviously China and America (and yeah, it's cheating, they're huge and have a ton of diversity). #3 is hard. I can see great arguments for like 5 different places. Japanese, Indian, Lebanese, Ethiopian, Mexican... idk man, that one is hard.